Some people like to mount partitions from other operating systems
(DOS, Windows, OS/2, etc.) so they can use them under Linux. These
partitions must be created and copied under their own operating system,
and you should unmount them before copying your Linux partition. For
example, if you have a DOS partition mounted at
/dos, you must unmount it with this
command:

umount /dos

Note that the command is umount, without the first
letter n in the word
unmount.